Powder metallurgy is the art of compacting powder to form a powder compact (hereinafter appropriately abbreviated as ‘a compact’) and sintering this compact to produce a sintered body. In this powder metallurgy, it is necessary to obtain a high density compact in order to obtain a sintered body with a high dimensional accuracy and a high density. To satisfy this need, it is necessary to increase compacting pressure for forming a compact.
As a method for producing a high density sintered body, a method comprising compacting twice and sintering twice, and powder metal forging have been carried out conventionally. These methods also need to obtain a high density compact in order to obtain a high density sintered body, and therefore, need to increase pressure for compacting powder.
In the case of applying a high compacting pressure, however, pressure for ejecting a compact from a die inevitably becomes high. When the ejecting pressure is high, there arise problems such as cracking and splitting of a compact and galling of a die. Therefore, the art of keeping the ejecting pressure low has been conventionally seeked for.
An example of this kind of art is to use a lubricant to reduce friction between a compact and a die in ejecting the compact. U.S. Pat. No. 4,955,798 discloses a warm compaction process in which powder and a die are heated to about 150° C. or less. This patent also discloses compaction carried out by using, as a lubricant to be mixed in powder, a metal stearate lubricant such as zinc stearate and lithium stearate or a wax lubricant in order to reduce pressure of ejecting a compact from a die. Japanese Unexamined Patent Publication (KOKAI) Nos. H05-271,709, H11-140,505, H11-100,602 and so on disclose methods of producing raw material powder containing a warm compaction lubricant and compaction methods using raw material powder containing a warm compaction lubricant. In addition, Japanese Unexamined Patent Publication (KOKAI) No. H8-100,203 discloses a method of applying a lubricant electrostatically to a die.
A study titled “INFLUENCE OF TEMPERATURE ON PROPERTIES OF LITHIUM STEARATE LUBRICANT” (Powder Metallurgy & Particulate Materials vol. 1, 1997) has been also published and this study discusses that when lithium stearate is used as a lubricant, as compaction temperature is higher, ejecting pressure is higher.
An iron-based sintered body has been demanded to have a higher density on the purpose of strength enhancement and volume reduction, and at the same time to attain higher dimensional accuracy and lower production costs. Accordingly, in order to obtain a high density sintered body by compacting and sintering only once, pressure for compacting powder must be high. In the conventional methods, however, an increase in compacting pressure accompanies a high ejecting pressure, which causes a problem that compaction cannot be continued because of degradation of compact surfaces and galling of a die.
Accordingly, it is an object of the present invention to provide a method of forming a powder compact which can produce a high density compact with a high compacting pressure and at the same time can reduce pressure for ejecting a compact from a die.